A surface mounted type resin mold package of QFP (QUAD FLAT PACKAGE), etc. becomes a problem that is important to prevent the package crack that occurred in the solder reflow process at the time of a surface mounting of the package. After molding a resin by the transfer mold method, some extent of moisture in the atmosphere has penetrated into the package because the resin that constitutes a package has absorbency. Therefore, heat at the time of a temperature cycle test after package completion, moisture in the resin is suddenly vaporized and expanded by heat at the time of soldering a package to the printed circuit board, and a package crack is caused.
As for a package crack that happened like this, it is known that it is especially easy to generate it in the vicinity of the interface between the back of a die pad on which a semiconductor chip is mounted and a resin surrounding it. This is because the adhesion strength of the metal that constitutes a lead frame and a resin is relatively weak, and especially because the die pad is the widest area in the lead frame sealed to the package, a resin breaks away locally from the back of the die pad by the reason why the heat stress that occurs at the time of a resin sealing process is large and moisture has accumulated in that portion.
The Japanese patent Laid-open No. Toku-Kai-Syou 63-204753 official gazette and the Japanese patent Laid-open No. Toku-Kai-Hei 6-216303 official gazette are proposing the lead frame structure that restrains an occurrence of a package crack, that is, they are proposing the lead frame structure which has the die pad whose outward appearance is smaller than that of the semiconductor chip mounted on it. According to this lead frame structure, an interface of a die pad and a resin becomes small, and the quantity of accumulated moisture also becomes small in the interface of them. And the occurrence of a package crack in the back vicinity of a die pad is restrained because a part of the back of a silicon wafer with good adhesion with a resin as compared with a lead frame comes to touch a resin and makes a direct interface.
On the other hand, as for the side of the main surface (element formation surface) of a chip mounted on a die pad, the surface passivation film (final passivation film) formed to the top layer part of a chip contacts with the resin that constitute a package. This final passivation film is made of inorganic system insulation materials such as the silicon oxide film or the silicon nitride film that formed by the method of a CVD (CHEMICAL VAPOR DEPOSITION), and the adhesion strength with the resin that constitutes a package is larger than that of a lead frame (metal) and a resin.
In case of the resin mold package that used the above lead frame structure which the outward appearance of a die pad is smaller than that of a chip mounted on it, the package crack in the interface vicinity of the back of a die pad and the resin is restrained, but the adhesion strength of the surface of a chip and a resin does not change.
Therefore, in case a package has a lot of moisture-absorption quantities, a package crack occurs on the side of the surface of a chip because an adhesion strength of the side of the surface of a chip relatively declined comparing to the side of the back of a die pad. And the serious defective as like as a breaking of wires was caused by these problems which has been clarified by the inventors.
The purpose of this invention is providing the technique that can improve reflow crack resistance of a resin package.
The foregoing and other purposes of this invention, and the new feature of this invention will be described with reference to the accompanying drawings.